Peripheral adrenergic receptors in hypertension

Ethnicity, gender, and age effects on adrenoceptors and physiological responses to emotional stress

We examined the unique and joint effects of ethnicity, gender, and age on cardiovascular and catecholamine responses to the anger recall interview, and beta(2)-adrenergic receptor density and function on peripheral blood mononuclear cells. Participants were 179 nonsmoking, normotensive men and women aged 18-49 years. All subjects showed similar blood pressure increases during the anger recall interview. Black men, however, showed the smallest increases in heart rate in conjunction with an attenuated peripheral vasodilatation. Black women and Whites showed similar increases in heart rate and peripheral vasodilatation. Increasing age was associated with greater norepinephrine increases to anger recall in Black males. Black men also exhibited higher epinephrine levels throughout the protocol, higher dissociation constant to (125)I-pindolol, and age-dependent increases in beta(2)-receptor density. Relative to Whites and Black females, arousal of negative affect in Black males led to a pattern of sympathetic nervous system activity that may help explain the higher prevalence of hypertension in this population.

Low arterial compliance in young African-American males

Hypertension remains a common public health challenge because of its prevalence and increase in co-morbid cardiovascular diseases. Black males have disproportionate pathophysiological consequences of hypertension compared with any other group in the United States. Alterations in arterial wall compliance and autonomic function often precede the onset of disease. Accordingly, our purpose was to investigate whether differences exist in arterial compliance and autonomic function between young, healthy African-American males without evidence of hypertension and age- and gender-matched non-African-American males. All procedures were carried out noninvasively following rest. Arterial compliance was calculated as the integrated area starting at the well-defined nadir of the incisura of the dicrotic notch to the end of diastole of the radial artery pulse wave. Power spectral analysis of heart rate and blood pressure variability provided distributions representative of parasympathetic and sympathetic modulations and sympathovagal balance. Baroreflex sensitivity (BRS) was calculated using the sequence method. Thirty-two African-American and twenty-nine non-African-American males were comparable in anthropometrics and negative family history of hypertension. t-Tests revealed lower arterial compliance (5.8 +/- 2.4 vs. 8.6 +/- 4.0 mmHg. s; P = 0.0017), parasympathetic modulation (8.9 +/- 1.1 vs. 9.7 +/- 1.1 ln ms2; P = 0.0063), and BRS (13.7 +/- 7.3 vs. 21.1 +/- 8.5 ms/mmHg; P = 0.0007) and higher sympathovagal balance (2.9 +/- 3.2 vs. 1.5 +/- 1.1; P = 0.03) in the African-American group. In summary, differences exist in arterial compliance and autonomic balance in African-American males. These alterations may be antecedent markers of disease and valuable in the detection of degenerative cardiovascular processes in individuals at risk.

Increased cardiac norepinephrine release in spontaneously hypertensive rats: role of presynaptic alpha-2A adrenoceptors

OBJECTIVES AND DESIGN

An increased sympathoadrenergic activation is thought to contribute to the maintenance of elevated blood pressure levels in hypertension. Therefore, the regulation of cardiac presynaptic sympathetic neurotransmission was investigated in spontaneously hypertensive (SHR) and Wistar-Kyoto rats (WKY).

METHODS AND RESULTS

Electrical field stimulation (1 min, 4 Hz) evoked a higher norepinephrine (NE) overflow from isolated perfused SHR than from WKY hearts (171 +/- 78 versus 111 +/- 27 nmol/g; means +/- SD, n = 7, P < 0.05). The difference in stimulation-evoked NE overflow was neither due to increased NE stores nor to a higher density of sympathetic nerve endings in SHR hearts. Furthermore, impairment of cardiac NE re-uptake was ruled out, as pharmacological inhibition of NE re-uptake by desipramine (300 nmol/l) similarly increased NE overflow from SHR (+ 54 +/- 17%) and WKY hearts (+ 59 +/- 18%). However, inhibition of presynaptic alpha-2 adrenoceptors (alpha-2R) with yohimbine (1 micromol/l) resulted in a significantly larger increase in NE overflow from WKY (+ 244 +/- 42%) than from SHR hearts (+ 162 +/- 47%, P < 0.05 versus WKY), indicating impairment of presynaptic inhibitory effect of alpha-2R in SHR. Supporting this notion, mRNA concentrations of alpha-2(A), the predominant presynaptic alpha-2R subtype, were reduced in SHR compared with WKY (738 +/- 251 versus 1468 +/- 518 mRNA molecules/10 ng, n = 7, P < 0.01), as quantified by competitive reverse transcription-polymerase chain reaction derived from left stellate ganglia.

CONCLUSIONS

The impairment of the alpha-2R mediated presynaptic negative feedback mechanism by a reduced expression of the alpha-2R subtype A may increase cardiac net secretion of NE in SHR and could therefore contribute to their hypertensive phenotype.

Effects of nonspecific beta-adrenergic stimulation and blockade on blood coagulation in hypertension

A hypercoagulable state might contribute to increased atherothrombotic risk in hypertension. The sympathetic nervous system is hyperactive in hypertension, and it regulates hemostatic function. We investigated the effect of nonspecific beta-adrenergic stimulation (isoproterenol) and blockade (propranolol) on clotting diathesis in hypertension. Fifteen hypertensive and 21 normotensive subjects underwent isoproterenol infusion in two sequential, fixed-order doses of 20 and then 40 ng. kg(-1). min(-1) for 15 min/dose. Thirteen subjects were double-blind studied after receiving placebo or propranolol (100 mg/day) for 5 days each. In hypertensive subjects, isoproterenol elicited a dose-dependent increase in plasma von Willebrand factor (vWF) antigen [F(2,34) = 5.02; P = 0.032] and a decrease in D-dimer [F(2,34) = 4.57; P = 0.040], whereas soluble tissue factor remained unchanged. Propranolol completely abolished the increase in vWF elicited by isoproterenol [F(1,12) = 10.25; P = 0.008] but had no significant effect on tissue factor and D-dimer. In hypertension, vWF is readily released from endothelial cells by beta-adrenergic stimulation, which might contribute to increased cardiovascular risk. However, beta-adrenergic stimulation alone may not be sufficient to trigger fibrin formation in vivo.

Beta(2)-adrenergic receptor gene delivery to the endothelium corrects impaired adrenergic vasorelaxation in hypertension

BACKGROUND

Impaired beta-adrenergic receptor (AR)-mediated vasorelaxation in hypertension plays a role in increased peripheral vascular resistance and blood pressure. Because the beta(2)AR is the most abundant vascular AR subtype, we sought to enhance betaAR vasorelaxation by overexpressing beta(2)ARs via adenoviral-mediated gene transfer (ADbeta(2)AR) to the vascular endothelium of the carotid artery.

METHODS AND RESULTS

In normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats, we exposed the right common carotid artery to ADbeta(2)AR in situ for 15 minutes by injection into the lumen while the blood flow was interrupted. Control carotids received an empty vector (ADempty). Three days later, transgene expression and selective endothelial localization were confirmed in infected vessels. Vasoregulation after beta(2)AR overexpression (2-fold) was studied in isolated organ baths. ADbeta(2)AR carotid responses to alpha(1)AR and alpha(2)AR agonists were not affected, whereas responses to epinephrine were altered and betaAR-mediated vasorelaxation was enhanced after beta(2)AR overexpression. As expected, betaAR-mediated vasodilatation in control carotids of SHR rats was significantly less than in similar control WKY carotid arteries. ADbeta(2)AR treatment enhanced betaAR vasorelaxation in SHR to levels similar to those seen in ADbeta(2)AR WKY carotids.

CONCLUSIONS

Our results demonstrate a critical role for the endothelium in betaAR-mediated vasorelaxation and suggest that impaired betaAR signaling may account for dysfunctional betaAR vasorelaxation in hypertension rather than impaired endothelium-dependent nitric oxide metabolism.

The effects of beta2 adrenoceptor gene polymorphisms on pressor response during laryngoscopy and tracheal intubation

We investigated whether human beta2 adrenoceptor (beta2AR) gene polymorphisms are associated with the pressor response to laryngoscopy and tracheal intubation. Ninety-two patients undergoing elective surgery under general anaesthesia were enrolled into this study. Arterial systolic pressure, heart rate and rate pressure product were measured before induction of anaesthesia and 1 min following laryngoscopy and tracheal intubation. Genomic DNA was then used to identify the beta2AR-16 and beta2AR-27 genes using an allele-specific polymerase chain reaction method. Using multiple linear regression models, controlling for age, sex, weight, baseline blood pressure, heart rate and rate pressure product, we found that patients who possessed the glutamic acid homozygote of beta2AR-27 produced significantly greater changes in mean arterial pressure and rate pressure products than patients with the glutamine homozygote of beta2AR-27 (beta coefficient for mean blood pressure = 11.81, beta coefficient for pulse-pressure product = 8.76, both p-values = 0.023). These findings suggest that genetic variability in the human beta2AR gene polymorphisms may be associated with the pressor response to laryngoscopy and tracheal intubation.

Peripheral blood mononuclear cell CD62L and CD11a expression and soluble interstitial cell adhesion molecule-1 levels following infused isoproterenol in hypertension

OBJECTIVES

Increasingly, studies indicate that alterations in leukocyte and endothelial cell adhesion molecules may enhance atherosclerotic processes in human hypertension. beta-adrenergic receptor activation has long been implicated in the aetiology and/or maintenance of hypertension and also has significant effects on leukocyte and endothelial adhesion molecules. This study therefore examined the effects of hypertension on peripheral blood mononuclear cell CD62L and CD11a expression and circulating soluble interstitial cell adhesion molecule (ICAM)-1 (sCD54) levels following infusion of the non-specific beta-adrenergic agonist isoproterenol.

DESIGN

In the setting of a General Clinical Research Center, 15 hypertensive and 20 normotensive subjects underwent an infusion of isoproterenol consisting of two sequential 15 min fixed-order doses of 20 and 40 ng/kg per min. Flow cytometry was used to quantify lymphocyte and monocyte populations and adhesion molecules, and ELISA was used to quantify sCD54 levels.

RESULTS

As expected, isoproterenol led to a significant increase in the number of circulating lymphocytes (P < 0.001) and monocytes (P < 0.01). The number of circulating CD3+CD8+CD62Llow T cytotoxic cells increased following isoproterenol (P < 0.001) and this increase was greater in hypertensives than in normotensives (P < 0.05). Isoproterenol led to a decrease in surface density of CD62L (P < 0.001) and an increase in surface density of CD11a (P < 0.001) in all subjects. Hypertensives had a significantly lower CD62L density (P = 0.01) and higher CD11a density on lymphocytes (P = 0.002) compared to normotensives. sCD54 levels were unchanged following isoproterenol but were elevated in hypertensives (P < 0.05).

CONCLUSIONS

A beta-adrenergic-induced environment of increased CD62Llow/CD11ahigh leukocytes, coupled with existing endothelial CD54 activation, could support basic atherosclerotic processes of increased peripheral blood mononuclear cell-endothelial adhesion in hypertension.

Peripheral adrenergic system and hypertension

Hypertension is a condition where adrenergic responsiveness, sympathetic activity and adrenoceptors are somewhat altered. Many techniques are available to assess human sympathetic nervous system activity. They each present limitations and disadvantages. Characterization and subdivision of the alpha and beta-adrenoceptors, according to their localization and answer to different agonists, was facilitated in recent years by the extensive use of pharmacological and molecular biology techniques. Some adrenoceptor studies were conducted on animal models, human tissues and peripheral blood cells to assess their changes in various forms and stages of hypertension. Our group has pointed out that alpha1-adrenergic receptors expressed by human peripheral blood lymphocytes underwent changes of density in essential hypertensives, compared to normotensive control subjects. The importance of these findings could provide an assessment of alpha1-peripheral receptors with possible future clinical implications in the pathophysiology and treatment of hypertension.

Sodium-sensitive hypertension is not associated with higher sympathetic nervous system activity in older hypertensive humans

The majority of older hypertensive humans are sodium sensitive and they are characterized by increased alpha-adrenergic responsiveness relative to their level of sympathetic nervous system (SNS) activity. To test the hypothesis that heightened SNS activity and/or increased alpha-adrenergic receptor responsiveness during sodium loading may play a role in the sodium-dependent increase in blood pressure in older sodium-sensitive hypertensives, we used compartmental analysis of [3H]norepinephrine (NE) kinetics to determine the release rate of NE into an extravascular compartment (NE2) as an index of systemic SNS activity and determined forearm blood flow responses to graded intrabrachial artery NE and angiotensin II (ANG II) infusions and platelet membrane alpha2-receptor properties in 24 older (age 64 +/- 7 years) hypertensive subjects. Subjects were studied at the end of 1 week of a low (20 mmol/day)- and again at the end of 1 week of a high (200 mmol/day)-sodium diet. Subjects were categorized as sodium sensitive (SS) if they had a > or = 5 mm Hg increase in mean arterial blood pressure (MABP) with dietary sodium loading (n = 16), or sodium-resistant (SR) if their MABP increased by < 5 mm Hg (n = 8). Neither dietary sodium intake nor sodium-sensitivity status significantly affected arterial plasma NE levels, NE2, or other NE kinetic parameters. Forearm blood flow responses to NE or to ANG II, and platelet alpha2-receptor properties were similar between the SS and SR groups. These results suggest that the sodium-dependent increase in MABP that characterizes SS hypertension among older humans is not because of an increase in systemic SNS activity or increased arterial adrenergic receptor responsiveness.

beta2-adrenoceptor gene polymorphisms and blood pressure variations in East Anglian Caucasians

OBJECTIVE

The amino-terminal polymorphisms, Arg16Gly and Gln27Glu, of the beta2-adrenergic receptor (beta2AR) have been shown to affect regulation of the receptor expression by an agonist in cell culture studies. The Arg16Gly polymorphism has also been recently shown to be associated with essential hypertension. We therefore evaluated whether the amino-terminal polymorphisms of beta2AR are associated with hypertension in a Caucasian population.

SUBJECTS AND METHODS

We performed an association study in 298 hypertensive patients and an equal number of age-matched normotensive controls from the East Anglian region, with blood pressure assessed categorically and quantitatively. We also examined the influence of the amino-terminal polymorphisms on blood pressure response to beta-blockade in 144 of the patients randomly assigned to this class of drug. Genotyping of the Arg16Gly polymorphism was undertaken by a newly designed mismatched polymerase chain reaction (PCR) and digestion with Nde I, whereas the Gln27Glu polymorphism was genotyped by PCR followed by Fnu4H I cleavage.

RESULTS

We found no differences in the genotype or allele frequencies of the beta2AR polymorphisms between hypertensive and normotensive participants. There was also no association between the beta2AR genotypes and variations in either basal blood pressure or the blood pressure response to a beta-blocker.

CONCLUSION

These findings suggest that the amino-terminal polymorphisms of the beta2AR gene are unlikely to constitute major susceptibility for essential hypertension in the East Anglian population.

Vascular β-adrenoceptor function in hypertension and in ageing

Functional β-adrenoceptors (β-AR) have been identified and characterized in blood vessels under in vivo conditions as well as in vascular smooth muscle cells (SMC) grown in culture. Agonist occupancy of β-AR activates adenylyl cyclase (AC) via the stimulatory guanine nucleotide-binding protein (Gs) and leads to elevations in intracellular adenosine 3',5'-cyclic monophosphate levels (cAMP). Increased cAMP activates the cAMP-dependent protein kinase (PKA), with subsequent phosphorylation of various target proteins. This β-AR pathway interacts with several other intracellular signalling pathways via cross-talk, so that activation by β-AR agonists may also modulate other second messengers and protein kinases. SMC β-AR play an important role in SMC function. In intact blood vessels they mediate SMC relaxation by various intracellular mechanisms, ultimately causing a decrease in intracellular Ca2+ levels. In cultured SMC, activation of the β-AR pathway results in inhibition of cellular proliferation, the development of SMC polyploidy, and SMC apoptosis. Blood vessels from hypertensive animals are characterized by an increase in SMC cell mass, a greater incidence of SMC polyploidy in the aorta, and an impairment in the β-agonist-mediated SMC relaxation. Some of these changes may result from an attenuation of β-AR function due to agonist-induced receptor desensitization caused by the uncoupling of receptors from the Gs-AC system. The phosphorylated β-AR may in turn trigger new signals and activate different intracellular pathways. However, the details of these mechanisms are still unresolved. Since functional β-AR play such a prominent and multi-faceted role in SMC function, it is important to understand how these diverse physiological effects are mediated by this receptor system, and how they contribute to the development of hypertension. With ageing, a decrease in β-AR-Gs-AC coupling is observed, and this is implicated in the reduced responsiveness of SMC. The similarities in SMC β-AR functional changes in hypertension and in ageing suggest that the underlying mechanisms are also analogous.Key words: smooth muscle, β-adrenoceptors, cyclic AMP, protein kinase A, cell proliferation, polyploidy, relaxation, apoptosis, hypertension, ageing.

The change of beta-adrenergic system in lead-induced hypertension

Lead exposure is considered to be a risk factor of cardiovascular disease. To investigate the relationship between lead and cardiovascular disease/hypertension in lead exposure, beta-adrenergic system is explored in this study. We address three topics in this study: (a) the relationship between beta-adrenergic receptor and lead level in heart, aorta, and kidney of lead-exposed rats; (b) the relationship between beta-adrenergic receptor in heart, aorta, kidney, and blood pressure in lead-exposed rats; and (c) the change of cyclic AMP level in heart, aorta, and kidney of rats with different lead levels. Wistar rats were chronically fed with 2, 1, 0. 5, 0.1, 0.05, and 0.01% lead acetate and water for 2 months. Plasma catecholamine level was measured by high-performance liquid chromatography. Radioligand binding assay was measured by a method that fulfilled strict criteria of beta-adrenoceptor using the ligand [(125)I]iodocyanopindolol. Cyclic AMP (cAMP) level was determined by radioimmunoassay. The levels of lead were determined by electrothermal atomic absorption spectrometry. The results showed that increased plasma norepinephrine level, decreased aorta beta-adrenergic receptor and cAMP, and increased kidney beta-adrenergic receptor and cAMP contributed to the elevation of blood pressure in lead-induced hypertension. The decrement of beta-adrenoceptor and cAMP in heart resulted in decreased contractility in heart.

Platelet alpha2-adrenoceptor alterations in patients with essential hypertension are normalized after treatment with doxazosin but not propranolol

OBJECTIVE

Marked alterations have been demonstrated to occur in the platelet alpha2-adrenoceptors of patients with essential hypertension. The purpose of this study was to determine whether antihypertensive treatment with alpha-adrenergic blocker doxazosin or beta-adrenergic blocker propranolol can affect the affinity and the density of platelet alpha2-adrenoceptors in such patients.

SUBJECTS AND METHODS

In two groups of 22 previously untreated, essential hypertensive patients, the mean affinity (Kd) and density (B(max)) of platelet alpha2-adrenoceptors were studied by [3H]-UK 14304 binding assays; the first assays were performed before any medication was begun, the second were performed after treatment for up to 13 weeks with doxazosin or propranolol. A third group of 22 healthy normotensive volunteers matched by age, sex and body mass index was used as control.

RESULTS

Blood pressure did not differ significantly in the two hypertensive groups, and treatment with the two drugs resulted in closely similar, normal blood pressure levels. Kd and B(max) values were significantly higher in the two hypertensive groups than in controls. After treatment with propranolol the binding parameters did not change significantly, whereas after treatment with doxazosin Kd and B(max) returned to normotensive values.

CONCLUSIONS

In previously untreated, essential hypertensive patients platelet alpha2-adrenoceptors have a lower affinity but a higher density than in normotensive subjects. Despite similar effects on blood pressure, the treatment with the alpha-adrenergic blocker doxazosin is followed by restoration of normal findings in the binding assays of platelet alpha2-adrenoceptors whereas the treatment with the beta-adrenergic blocker propranolol does not alter the Kd and B(max) values.

Functional correlates of alpha(2A)-adrenoceptor gene polymorphism in the HANE study

BACKGROUND

The aim of this study was to test an association between alleles of the alpha(2A)-adrenoceptor gene with hereditary hypertension and with alterations of lipid and carbohydrate metabolism.

METHODS

Genomic DNA was isolated from 147 hypertensive patients and digested with DraI. Genotypes at the alpha(2A)-adrenoceptor were identified by restriction fragment length polymorphism. Genotype at each locus was related to blood pressure, family history of hypertension and various clinical chemistry parameters.

RESULTS

The alpha(2A)-adrenoceptor polymorphism was not significantly associated with blood pressure or a family history of hypertension. Patients with the d allele of the alpha(2A)-adrenoceptor had significantly lower HbA(1) (5.6 vs 6.2%, P=0.0344) and HbA(1c) (3.4 vs 3.9%, P=0.0237) and total cholesterol (212 vs 229 mg/dl, P=0.0333) than those without. Similar trends, which failed to reach statistical significance, were seen for glucose, triglycerides and LDL cholesterol.

CONCLUSIONS

We propose that alleles at the alpha(2A)-adrenoceptor locus might contribute to interindividual differences in the regulation of human lipid and glucose metabolism.

Sympathetic and respiratory responses to hypoxia in essential hypertension

An increased sympathetic nervous response to hypoxia and reduced beta-adrenergic receptor function have been reported in hypertension. This study examines the relationship between hypoxia and beta-adrenergic receptor function in sixteen normotensive and eight hypertensive subjects. We measured the average arterial oxygen saturation and the end tidal carbon dioxide partial pressure in hypertensive and normotensive groups under normoxia and mild isocapnic hypoxia (15% O2, 85% N2). The ratio of isoproterenol-stimulated cAMP to basal cAMP on lymphocytes was measured in the two groups under normoxia. We also measured plasma norepinephrine levels and calculated the "Chronotropic 25 Dose" of isoproterenol in the two groups under normoxia and hypoxia. Hypertensives had higher plasma norepinephrine levels for either breathing condition (F=7.16, p=0.015). Under hypoxia, hypertensives showed a significant decrease in the average arterial oxygen saturation (F=4.92, p=0.038) and higher "Chronotropic 25 Dose" implying decreased beta-adrenergic receptor sensitivity (F=6.30, p=0.011). These results suggest that hypertensives have a diminished ventilatory response and impaired beta-adrenergic response under hypoxia.

Sympathetic nervous system activity and alpha-adrenergic responsiveness in older hypertensive humans

We have previously demonstrated in normotensive humans an age-associated increase in sympathetic nervous system (SNS) activity combined with appropriate downregulation of alpha-adrenergic responsiveness. Impaired downregulation of alpha-adrenergic responsiveness, despite a comparable level of SNS activity, could contribute to higher blood pressure in older hypertensive humans. We measured arterial plasma norepinephrine (NE) levels and the extravascular NE release rate (NE2) derived from [3H]NE kinetics (to assess systemic SNS activity), and platelet and forearm arterial adrenergic responsiveness in 20 normotensive (N) and in 24 hypertensive (H), otherwise healthy, older subjects (60-75 yr). Although plasma NE levels were similar (N 357 +/- 27 vs. H 322 +/- 22 pg/ml; P = 0.37), NE2 tended to be greater in the hypertensive group (H 2.23 +/- 0.21 vs. N 1.64 +/- 0.20 microgram. min-1. m-2; P = 0. 11), and the NE metabolic clearance rate was greater (H 1,100 +/- 30 vs. N 900 +/- 50 ml/m2; P = 0.004). In the hypertensive group, there was a greater alpha-agonist-mediated inhibition of platelet membrane adenylyl cyclase activity and a NE- but not ANG II-mediated decrease in forearm blood flow. Compared with normotensive subjects, in older hypertensive subjects 1) NE metabolic clearance rate is increased, 2) systemic SNS activity tends to be increased, and 3) arterial and platelet alpha-adrenergic responsiveness is enhanced. These results suggest that heightened SNS activity coupled with enhanced alpha-adrenergic responsiveness may contribute to elevated blood pressure in older hypertensive humans.

Platelet alpha2-adrenoceptor alterations in patients with essential hypertension

AIMS

The purpose of this study was to determine whether human platelet alpha2-adrenoceptors were altered in essential hypertension. A systematic analysis was carried out on 165 normotensives and 124 untreated primary hypertensives.

METHODS

The study was performed at different levels: i) density and affinity of platelet alpha2-adrenoceptors were determined by receptor binding assays using the full alpha2-adrenoceptor agonist [3H]-UK 14304 and a thermodynamic analysis of data was carried out to evaluate if binding mechanisms at the molecular level were altered during hypertension; ii) the functionality of Gi proteins coupled to alpha2-adrenoceptors and iii) forskolin-stimulated cAMP levels were measured.

RESULTS

Platelet alpha2-adrenoceptors mean density (Bmax) and affinity (Kd) (+/-s.e.mean) were significantly lower and higher, respectively, in normotensive than in hypertensive subjects [Bmax=327+/-4 vs 435+/-5 fmol mg(-1) of protein (P<0.01) and Kd=3.76+/-10.05 vs 6.50+/-0.15 nM (P<0.01), respectively]. The 50% stimulating concentration of adrenaline on [35S]-GTPgammaS binding to Gi proteins was significantly (P<0.01) lower in normotensives (12+/-2 nM) than in hypertensives (110+/-10 nM). The 50% inhibiting concentration of adrenaline on forskolin-stimulated cAMP levels was significantly (P<0.01) lower in normotensive (22+/-2 nM) than in hypertensive subjects (200+/-25 nM).

CONCLUSIONS

Present analysis, including receptorial and functional data, provides evidence that marked alterations occur in platelet alpha2-adrenoceptors of hypertensive subjects.

Bbeta-adrenergic receptor kinase-1 levels in catecholamine-induced myocardial hypertrophy: regulation by beta- but not alpha1-adrenergic stimulation

Pressure overload ventricular hypertrophy is accompanied by dysfunctional beta-adrenergic receptor signaling due to increased levels of the beta-adrenergic receptor kinase-1, which phosphorylates and desensitizes beta-adrenergic receptors. In this study, we examined whether increased beta-adrenergic receptor kinase 1 expression is associated with myocardial hypertrophy induced by adrenergic stimulation. With use of implanted mini-osmotic pumps, we treated mice with isoproterenol, phenylephrine, or vehicle to distinguish between alpha1- and beta-adrenergic stimulation. Both treatments resulted in cardiac hypertrophy, but only isoproterenol induced significant increases in beta-adrenergic receptor kinase-1 protein levels and activity. Similarly, in isolated adult rat cardiac myocytes, 24 hours of isoproterenol stimulation resulted in a significant 2.8-fold increase in beta-adrenergic receptor kinase-1 protein levels, whereas 24 hours of phenylephrine treatment did not alter beta-adrenergic receptor kinase-1 expression. Our results indicate that increased beta-adrenergic receptor kinase-1 is not invariably associated with myocardial hypertrophy but apparently is controlled by the state of beta-adrenergic receptor activation.

Dissociation of blood pressure reduction from end-organ damage in TGR(mREN2)27 transgenic hypertensive rats

OBJECTIVE

Since the biochemical disturbance underlying hypertension may be an important determinant of patient outcome, we compared the effects of early treatment with different antihypertensive drugs on end-organ damage in the TGR(mREN2)27 transgenic rat (REN-2). In these REN-2 rats, hypertension is primarily caused by increased activity of the tissue renin-angiotensin system.

DESIGN AND METHODS

Seven-week-old REN-2 rats were either untreated or treated orally with an optimal daily dose of carvedilol (30 mg/kg), hydralazine (30 mg/kg), losartan (10 mg/kg) or quinapril (15 mg/kg). Nontransgenic littermates served as normotensive controls. After 11 weeks of treatment, we determined plasma norepinephrine concentrations, left ventricular atrial natriuretic factor messenger RNA and cardiac and vascular function and hypertrophy.

RESULTS

Chronic treatment with carvedilol and hydralazine significantly decreased blood pressure to a similar level but failed to normalize it, whereas both losartan and quinapril completely normalized blood pressure. Despite a blood pressure reduction in all treatment groups, only losartan, quinapril and hydralazine preserved endothelial function, while carvedilol did not. Furthermore, losartan and quinapril prevented cardiac and medial hypertrophy. The expression of atrial natriuretic factor messenger RNA paralleled the hemodynamic changes. Plasma norepinephrine levels were normalized by losartan or quinapril but remained increased after carvedilol and hydralazine treatment.

CONCLUSIONS

In REN-2 hypertensive rats, end-organ damage can be prevented by both inhibition of the angiotensin converting enzyme and blockade of the angiotensin II type 1 receptor, but not by merely lowering blood pressure. When blood pressure is not fully normalized, the effects on end-organs are clearly dissociated from the antihypertensive effects.

Impaired function of alpha-2 adrenoceptors in smooth muscle of mesenteric arteries from spontaneously hypertensive rats

The alpha2-adrenoceptor function in mesenteric arteries of spontaneously hypertensive rats (SHR) was investigated by comparing membrane potential changes in response to adrenergic agonists in preparations from female SHR, Wistar-Kyoto (WKY) and normotensive Wistar rats (NWR). Resting membrane potential was found to be less negative in mesenteric arteries from SHR than in those from NWR and WKY. Apamin induced a decrease in the membrane potential of mesenteric artery rings without endothelium from NWR and WKY, but had no effects in those from SHR. Both UK 14,304 and adrenaline, in the presence of prazosin, induced a hyperpolarization that was significantly lower in de-endothelialized mesenteric rings from SHR than in those from NWR and WKY. In mesenteric rings with endothelium, however, similar hyperpolarization was observed in the three strains. In NWR mesenteric rings with endothelium the hyperpolarization induced by activation of alpha2-adrenoceptors was abolished by apamin, whereas in intact SHR mesenteric rings this hyperpolarization was slightly reduced by apamin and more efficiently reduced by Nomega-nitro-L-arginine. It is concluded that the activity of potassium channels coupled to alpha2-adrenoceptors is altered in the smooth muscle cells of SHR mesenteric arteries, contributing to their less negative membrane potential. On the other hand, the endothelial alpha2-receptors are functioning in mesenteric vessels from SHR and their stimulation induces a hyperpolarization mainly through the release of nitric oxide.

Chamber-specific alterations of norepinephrine uptake sites in cardiac hypertrophy

The present study investigated local differences of sympathetic activation and sympathetic neuroeffector defects in nonhypertrophied right and hypertrophied left ventricles in a rat model with renin-induced pressure overload [TG(mREN2)27]. As judged from the depletion of myocardial norepinephrine stores, sympathetic activation was more pronounced in the left than in the right ventricles. In addition, norepinephrine uptake1 carrier sites were reduced in left but unchanged in right ventricles. Gene expression of the carrier was unchanged in stellate ganglia. An increase of Gialpha expression and a heterologous adenylyl cyclase desensitization occurred only in the left but not in the right ventricles, whereas a reduction of beta-adrenergic receptors was observed in both chambers. We concluded that general sympathetic activation can lead to beta-adrenoceptor downregulation but that pressure overload further increases sympathetic activation involving norepinephrine uptake mechanisms in the left ventricles, resulting in heterologous beta-adrenergic desensitization.

Adrenaline responsiveness in mild hypertension: no evidence for altered beta-adrenoceptor sensitivity

The effects of circulating adrenaline on cardiovascular function were studied in 14 subjects (mean age, 36.5 years; range, 19-46 years) with mild hypertension and in 14 normotensive controls, matched for age and sex. Adrenaline was infused i.v. in step-wise increasing doses (0.1, 0.2, 0.4, and 0.8 nmol/kg/min). Cardiovascular responses were evaluated by echocardiography and noninvasive blood pressure measurements. Noradrenaline, adrenaline, potassium, and cyclic adenosine monophosphate (cAMP) were determined in venous plasma. Systolic and diastolic blood pressure responses to adrenaline were similar in both groups. Adrenaline increased myocardial contractility and stroke volume, but less so in the hypertensive patients. Cardiac output was increased in the hypertensive patients at rest, but the signs of increased myocardial contractility disappeared during adrenaline infusion, most likely because of a reduced myocardial compliance. Increased heart rate and systemic vascular resistances were displayed by the hypertensive patients at all adrenaline concentrations studied, but the responses were similar in both groups. The adrenaline-induced decreases in potassium and increases in cAMP were also similar in both groups. The increases in myocardial contractility and in heart rate are compatible with an increased arousal in mild hypertension at rest. Mild hypertension does not appear to be associated with alterations of beta2-adrenoceptor sensitivity, and the findings do not support that adrenaline is involved in the pathogenesis of primary hypertension.

Effects of angiotensin II type 1 receptor blockade and angiotensin-converting enzyme inhibition on cardiac beta-adrenergic signal transduction

Inhibition of the renin-angiotensin system has been shown to improve symptoms and prognosis in heart failure. We compared the effects of inhibition of angiotensin-converting enzyme or blockade of angiotensin II type 1 (AT1) receptors in a model with renin-induced hypertension that is known to exhibit similar changes in sympathetic activation and beta-adrenergic desensitization, as observed in heart failure. Treatment with captopril (100 mg/kg of feed) or the AT1-antagonist Bay 10-6734 (100 mg/kg of feed) was performed in transgenic rats harboring the mouse renin 2d gene [TG(mREN2)27]. Neuropeptide Y and angiotensin II levels, adenylyl cyclase activity, beta-adrenergic receptors, G(salpha), and G(ialpha) were investigated. TG(mREN2)27 showed a depletion of myocardial neuropeptide Y stores and an increase in myocardial angiotensin II concentrations. Isoprenaline- and guanylylimidodiphosphate-stimulated adenylyl cyclase activities and beta-adrenergic receptor density were reduced, whereas the catalyst and G(salpha)-function were unchanged. G(ialpha) protein and mRNA concentrations were increased. All alterations were normalized by both treatments. Systolic left ventricular pressures, plasma atrial natriuretic peptide, and myocardial steady state atrial natriuretic peptide mRNA concentrations and heart weights were similarly reduced by both treatments. Sympathetic neuroeffector defects are similarly reversed by angiotensin-converting enzyme inhibition or AT1 antagonism. The data support the concept that pharmacological interventions in the myocardial renin-angiotensin system significantly reverse local sympathetic neuroeffector defects. This could be important for the beneficial effects of these agents.

Sedative and analgesic effects of medetomidine in beagle dogs infected and uninfected with heartworm

The sedative and analgesic effects of medetomidine were evaluated in heartworm-infected (HW+) and uninfected (HW-) beagle dogs by intravenous (IV) and intramuscular (IM) administration of 30 micrograms/kg and 40 micrograms/kg doses, respectively. Posture, response to noise and the pedal reflex were monitored. A procedure for mock radiographic positioning was performed to evaluate its overall clinical use. Observation times were 0, 15, 30, 60, 90, 120 and 180 min. In addition, the times from injection until the dog could not stand on its feet (down time), from lateral to sternal recumbency (sternal recumbency time), and from sternal recumbency to rising again (rising time) were also noted. Medetomidine produced rapid sedation and analgesia by both routes. Down times for the IM and IV routes were similar, which verified the manufacturer's recommended doses. The HW+ dogs had shorter down times, probably owing to increased blood flow to the brain caused by adrenergic alpha-2 activity. Sternal recumbency and rising times did not differ between the groups, suggesting a similar metabolism. Sedation and analgesia were adequate for performing the procedure in all dogs. HW- dogs showed less resistance to handling during the procedure than HW+ dogs. Overall, medetomidine seems to be a suitable agent for short-term chemical restraint in dogs, even with subclinical heartworm infestation.

Synthesis and evaluation of (S)-4-(3-(2'-[11C]isopropylamino)-2-hydroxypropoxy) -2H-benzimidazol -2-one ((S)-[11C]CGP 12388) and (S)-4-(3-((1'-[18F]-fluoroisopropyl)amino)-2-hydroxypropoxy) -2H- benzimidazol-2-one ((S)-[18F]fluoro-CGP 12388) for visualization of beta-adrenoceptors with positron emission tomography

The beta-adrenoceptor antagonist (S)-[11C]CGP 12177 (4-(3-(tert-butylamino)-2-hydroxypropoxy)-2H-benzimidazol -2[11C]- one) is a generally accepted radioligand for cardiac and pulmonary PET studies. The synthesis of [11C]CGP 12177 is a laborious and often troublesome procedure. Therefore, (S)-CGP 12388 (4-(3-(isopropylamino)-2-hydroxypropoxy) -2H-benzimidazol-2-one), 5, the isopropyl analogue of CGP 12177, has been labeled with carbon-11 in the isopropyl group via a reductive alkylation by [11C]acetone (3) of the corresponding (S)-desisopropyl compound 2. The fluoro-substituted analogue of (S)-CGP 12388 was prepared by reacting 2 with [18F]fluoroacetone (4). (S)-[11C]CGP 12388 (5) was easily prepared via a one-pot procedure. The radiochemical yield of (S)-[11C]CGP 12388 (600-800 Ci/mmol, EOS) was 18% (EOB) with a total synthesis time of 35 min, whereas (S)-[18F]fluoro-CGP 12388 (6) (> 2000 Ci/mmol, EOS) was synthesized in 105 min with a radiochemical yield of 12% (EOB). Biodistribution studies in rats demonstrated specific binding to beta-adrenoceptors of (S)-[18F]fluoro-CGP 12388 and (S)-[11C]CGP 12388 in lung and heart. The lungs were clearly visualized with PET studies of rats. Total/nonspecific binding at 60 min postinjection was 5.6 for (S)-[11C]CGP 12388 and 2.0 for the (S)-18F compound. Due to its facile synthetic procedure and in vivo data, (S)-[11C]CGP 12388 is a promising beta-adrenoceptor ligand for clinical PET.

Spontaneously hypertensive rat: cholera toxin converts suppression to immunity through a Th2 cell-IL-4 pathway

The spontaneously hypertensive rat (SHR) exhibits a number of T cell dysfunctions that develop concurrently with elevated blood pressure. Studies have shown a mitogen-induced lymphocyte suppression mediated in part by the production of interferon-gamma (IFN-gamma), which stimulated NO production by macrophages. To assess whether this immune suppression is reversible, SHR were immunized with diphtheria toxoid (DT) with or without cholera toxin (CT) as adjuvant. SHR immunized with DT only displayed weak serum immunoglobulin G (IgG) anti-DT titers, tenfold less than similarly treated normotensive Wistar-Kyoto rats (WKYR). SHR CD4+ T cells failed to proliferate upon in vitro stimulation with DT. In contrast, SHR coimmunized with DT and CT showed serum IgG antibody titers similar to WKYR and Brown Norway rats. Coimmunization with CT rescued SHR CD4+ T cells from suppression and supported DT- or B subunit of CT-specific proliferative responses, and these cells produced more interleukin-4 (IL-4) than IFN-gamma, and anti-IFN-gamma antibody treatment enhanced IL-4 production. Exogenous IL-4 increased the proliferation of antigen-specific CD4+ T cells, whereas IFN-gamma was inhibitory. This study shows that the adjuvant CT induces T helper 2-type responses, reversing the T cell dysfunction in the SHR.

Effects of adrenaline infusion on plasma lipids and noradrenaline levels in rabbits with adriamycin-induced cardiomyopathy

1. We investigated the acute effects of adrenaline infusion on plasma lipid levels in vehicle- and adriamycin-treated rabbits. Lipids were measured before and 30 and 60 min after the commencement of continuous intravenous administration of adrenaline (0.06 microgram/kg per min) or saline in pentobarbital-anaesthetized rabbits. 2. Adrenaline infusion significantly increased plasma free fatty acid (P < 0.05) and noradrenaline (NA) levels (P < 0.05) in vehicle-treated control rabbits, but not in adriamycin-treated rabbits. However, adrenaline had no effect on plasma total cholesterol, free cholesterol, high-density lipoprotein-cholesterol, triglyceride or phospholipid levels. 3. Pretreatment with propranolol almost completely inhibited increased plasma free fatty acid and NA levels associated with adrenaline infusion, suggesting that adrenaline increases plasma free fatty acid and NA levels via the stimulation of beta-adrenoceptors in vehicle-treated rabbits. 4. It is suggested that both the production of plasma free fatty acids and the release of NA via the activation of beta-adrenoceptors is reduced in rabbits with adriamycin-induced cardiomyopathy. This may be related to the down-regulation of beta-adrenoceptors caused by elevated plasma NA levels induced by cardiac failure.

Mechanical load opposes angiotensin-mediated decrease in vascular alpha 1-adrenoceptors

alpha 1-Adrenergic receptor contraction of vascular smooth muscle is augmented by increases in angiotensin II and also in several forms of hypertension. Whether angiotensin directly modulates alpha 1-adrenoceptor subtype expression to contribute to this effect is unknown. In a previous study, we demonstrated that increased mechanical load (pressure) per se does not alter expression of alpha 1B- and alpha 1D-adrenoceptors in rat aortic smooth muscle in cell culture, in vitro or in vivo. However, findings in aortic coarctation hypertension suggested that a humoral factor, possibly angiotensin, selectively reduces alpha 1B-adrenoceptors and that increased mechanical load opposes this decrease. The present study examined this hypothesis by determining the effect of angiotensin alone and in the presence of mechanical loading on the expression of alpha 1D- and alpha 1B-adrenergic receptor mRNAs and alpha 1-receptor density in cultured aortic smooth muscle cells. alpha 1D mRNA content, per smooth muscle cell, concentration-dependently decreased after 3 hours of exposure to 0.3 nmol/L to 1 mumol/L angiotensin but by 24 hours had returned to control levels. In contrast, alpha 1B mRNA concentration-dependently declined at a later time (24 hours) and remained decreased at 48 hours to 27 +/- 6% of control with 1 mumol/L angiotensin. Angiotensin also decreased alpha 1-adrenoceptor density in a dose-dependent manner. Angiotensin had no effect on cell number in these confluent, quiescent cells but did increase cell protein and total RNA. This cellular hypertrophy and the decreases in alpha 1-adrenoceptor mRNAs were blocked by the angiotensin type 1 receptor antagonist losartan. Cyclic mechanical loading of smooth muscle cells opposed the angiotensin-mediated hypertrophy and decrease in alpha 1B mRNA expression and alpha 1-adrenergic receptor density. These data suggest that angiotensin and intravascular pressure interact to affect cell growth and expression of alpha 1B-adrenergic receptors by vascular smooth muscle.

Plasma catecholamine and lymphocyte beta 2-adrenergic receptor alterations in elderly Alzheimer caregivers under stress

OBJECTIVE

The purpose of this study was to determine the effects of chronic stress on beta-adrenergic physiology in elderly spousal caregivers to Alzheimer patients.

METHODS

Thirty-seven elderly spousal caregivers and matched noncaregiver controls (mean age 73 years, SD = 6) were studied. Life stress categorization (presence of marked threat) covering the previous 6 months was determined using a semistructured interview based on the Psychiatric Epidemiological Research Inventory and the Life Events and Difficulties Schedule. beta 2-adrenergic receptor sensitivity (isoproterenol-stimulated cyclic AMP accumulation) and density were determined in lymphocytes.

RESULTS

Caregivers with high life stress had higher plasma norepinephrine levels (p < .04) but no change in plasma cortisol. For beta-receptor sensitivity, 30% of the variance was accounted for by high life stress rating, increased age, being male, and lower norepinephrine (p = .018); 17% of the variance in beta-receptor density was accounted for by plasma norepinephrine (p = .03).

CONCLUSIONS

The findings demonstrate that chronic high stress may be associated with changes in adrenergic physiology and may provide a mechanism through which chronic stress alters cellular immunity.

Short-term aortic barodenervation diminishes alpha 1-adrenoceptor reactivity in rat aortic smooth muscle

Our previous studies have shown that aortic baroreceptor denervation elicits acute increases in blood pressure and significant elevations of sympathetic activity and peripheral vascular resistance. This study investigated the short-term (3 and 48 h) effect of aortic barodenervation and associated sympathetic hyperactivity on the functional activity of alpha 1-adrenoceptors in rat aortic smooth muscle. Compared with sham operation, aortic barodenervation caused acute rises in blood pressure and heart rate and reductions in baroreflex sensitivity. Blood pressure and heart rate remained elevated when measured in conscious aortic barodenervated rats 3 h after surgery but subsided to sham-operated levels at 48 h; the baroreflex sensitivity, however, remained attenuated. Hexamethonium (0.5-4 mg/kg, i.v.) elicited significantly (P < 0.05) greater depressor responses in conscious aortic barodenervated rats than in sham-operated rats at both 3 and 48 h, suggesting a higher sympathetic activity in denervated rats. Exposure of aortic rings from aortic barodenervated and sham-operated rats to cumulative addition of phenylephrine (alpha 1-adrenoceptor agonist, 3 x 10(-8)-1 x 10(-4) M) resulted in concentration related contractile responses that were similar in the two groups of rats at 3 h in contrast to significantly (P < 0.05) smaller contractions in rings from denervated rats at 48 h. The maximum contraction developed (Emax) at 48 h showed approximately 50% reduction in rings from aortic barodenervated compared with sham-operated rats (239 +/- 16 vs. 558 +/- 15 mg tension/mg tissue). The pA2 value for prazosin (alpha 1-adrenoceptor antagonist) was not altered by aortic barodenervation at 3 h but showed significant (P < 0.05) increases, compared with sham-operated values, at 48 h. It is concluded that short-term aortic barodenervation results in an elevation of sympathetic activity that coincides with reduced responsiveness of aortic smooth muscle to alpha 1-adrenoceptor activation. The aortic barodenervation-induced alpha 1-adrenoceptor desensitization is not a result of decreased receptor affinity but may involve an alteration of receptor density or in the post-receptor activation events.

The cardiac beta-adrenoceptor-mediated signaling pathway and its alterations in hypertensive heart disease

Hypertension-induced cardiac hypertrophy is a predictor of the development of cardiac failure. It is unknown which cellular markers contribute to the progression from compensated hypertrophy to failure. In heart failure, several signal transduction defects leading to adenylate cyclase desensitization have been demonstrated, such as beta-adrenoceptor downregulation, increase of inhibitory G protein expression, and uncoupling of beta-adrenergic receptors, presumably by an increase of receptor kinase activity. In hypertensive heart disease, most studies have been performed in rat models of hypertension. As in heart failure, heterologous adenylyl cyclase desensitization occurs. The mechanisms are often different between the heterogeneous models for acquired and genetic hypertension, but Gi protein alterations and beta-adrenoceptor downregulation have been observed frequently. The underlying mechanism for desensitization is most likely a sympathetic activation in established hypertension rather than genetic alterations of signal transduction proteins. The data available suggest that beta-adrenergic desensitization could represent a mechanism that contributes to the progression from hypertrophy to failure. The key question remains whether those hypertensive patients who develop heart failure are more prone to beta-adrenergic desensitization or whether early intervention to reduce sympathetic activity is more effective in preventing or delaying the transition from compensated hypertrophy to overt failure.

Influence of adrenodemedullation on beta 2- and beta 3-adrenoceptors mediating relaxation of oesophageal smooth muscle of spontaneously hypertensive rats

1. In oesophageal smooth muscle strips from spontaneously hypertensive rats (SHR) of 8-10 and 22-24 weeks of age, respectively, beta-adrenoceptor-mediated relaxation was investigated, by use of the beta-agonists, (-)-isoprenaline and fenoterol (both in the absence and presence of the beta 2-selective antagonist ICI 118,551) and the selective beta 3-agonist, BRL 37,344. 2. In preparations from 8-10 week SHR, (-)-isoprenaline- and fenoterol-induced concentration-response curves (CRCs) were hardly antagonized by ICI 118,551 at concentrations up to 1 microM, indicating only a minor contribution of beta 2-adrenoceptors. pA2-values for ICI 118,551 of 5.30 ((-)-isoprenaline as agonist) and 5.46 (fenoterol as agonist), estimated from the shifts at the highest (10-100 microM) antagonist concentrations, are consistent with affinity at a beta 3-adrenoceptor, similar to that in Wistar rat oesophageal smooth muscle. 3. In 8-10 week SHR, adrenodemedullated at 4 weeks of age (SHR-ADM4) the potency of fenoterol was markedly increased and CRCs were shallow. In addition, ICI 118,551 (0.1 microM) now produced a clear rightward shift accompanied by a steepening of the CRC. A marked further shift was observed only at 100 microM of the antagonist. The data are compatible with the involvement of both beta 2- and beta 3-adrenoceptors. 4. In 22-24 week animals, the same differences between SHR and SHR-ADM4 were observed with fenoterol as in 8-10 week animals, though beta-adrenoceptor responsiveness was slightly decreased. The potency of ICI 118,551 at beta 3-adrenoceptors (pA2 = 5.11) was significantly different from the pA2 value of 5.46 obtained with the younger animals. 5. Responses to the beta 3-adrenoceptor agonist, BRL 37,344, were similar in Wistar rat and SHR preparations. In 8-10 week SHR, a small decrease in the maximal response was observed, which in animals of 22-24 weeks of age was accompanied by a small decrease in the pEC50 value as well. 6. The results clearly indicate that beta 2-adrenoceptors in SHR oesophageal muscularis mucosae are desensitized, whereas beta 3-adrenoceptor-mediated responses are unaffected and similar to the responses observed in the Wistar rat oesophagus. The functional presence of beta 2-adrenoceptor-responses in SHR-ADM4 suggests a major role for adrenal-derived adrenaline in the desensitization of the beta 2-adrenoceptor-population.

Heterotrimeric G proteins in heart disease

Heterotrimeric G proteins couple many types of cell surface receptors to intracellular effectors such as enzymes or ion channels. In the mammalian heart, G protein-mediated signalling pathways are involved in the regulation of contractile force, heart rate, conduction velocity, and relaxation. In the first part of this review we summarize some important structural and functional features of receptors, G proteins, and effectors with special focus on the heart. In the second part, we review the current knowledge about alterations of G protein-mediated signalling in heart disease such as myocardial hypertrophy and heart failure.

Association of hypertension with beta2- and alpha2c10-adrenergic receptor genotype

The adrenergic receptors have been implicated in the pathogenesis of essential hypertension. We hypothesized that hypertension is associated with variants at the beta2-adrenergic receptor locus and at one of the alpha2-adrenergic receptor loci. In unrelated individuals, we measured untreated blood pressure and characterized each subject as hypertensive or normotensive. We then used genomic DNA to identify beta2- and alpha2c10-adrenergic receptor restriction fragment length polymorphisms. In 175 subjects (49 percent with hypertension, 55 percent black), both hypertension and race were associated with genotype at the beta2 locus (chi2 for hypertension = 11, P = .004, chi2 for race = 8.8, P = .012). The association with hypertension persisted in each race group separately (blacks only: chi2 = 9.6, P = .008; whites only; chi2 = 14.2, P = .001). This association persisted in a logistic model that controlled for race (P = .01). Genotype was also significantly associated with baseline systolic, diastolic, and mean arterial blood pressures (P = .05, .01, and .02, respectively). These data suggest that the beta2-adrenergic receptor gene is a candidate gene for hypertension in blacks and whites. We also genotyped subjects at the alpha2-adrenergic receptor coded on chromosome 10. There was no association between hypertension and genotype at the alpah2c10 locus in the total group or in blacks, but there was significant association in whites (chi2 = 6.7, P = .03). These data suggest that the beta2- and alph2c10-adrenergic receptor genes may contribute, in a race-specific manner, to the inheritance of essential hypertension. Linkage studies in related individuals are needed to confirm these findings.

Noradrenergic transmission in the tail artery of hypertensive rats transgenic for the mouse renin gene Ren-2

1. The aim of the present study was to analyse the noradrenergic transmission in the tail artery of hypertensive rats transgenic for the mouse renin gene Ren-2 (TGR) in comparison with its control, the Sprague-Dawley (SD) rat. 2. Electrical field stimulation (EFS) of vascular segments produced frequency-dependent vasoconstrictions that were significantly greater in TGR arteries. 3. These contractions were abolished by tetrodotoxin (0.1 microM). Phentolamine (50 nM) and prazosin (1 - 10 nM) produced an inhibition of these responses that was significantly greater in SD arteries, whereas that produced by yohimbine (0.5-1 microM) was higher in TGR arteries. In both strains, propranolol (1 microM) potentiated the responses to EFS, and this increase was observed at lower frequencies in TGR arteries. 4. The EFS-evoked [3H]-noradrenaline (NA) release was significantly greater in TGR than in SD rats. However, NA (10 nM-10 microM) reduced and yohimbine and phentolamine (10 nM-10 microM) increased the tritium outflow to a similar degree in both strains. 5. Exogenous NA also induced greater vasoconstriction in TGR arteries. 6. These results suggest the existence in TGR tail artery of an increase in: (a) NA-release and alpha 2-adrenoceptor-mediated contractions, which could contribute to the elevated blood pressure in these rats; and (b) beta-adrenoceptor-mediated vasodilatations, which may be a mechanism to counteract high blood pressure.

(S,S)- and (S,R)-1'-[18F]fluorocarazolol, ligands for the visualization of pulmonary beta-adrenergic receptors with PET

The beta-adrenoceptor antagonist carazolol has been labelled with fluorine-18 in the isopropyl group via a reductive alkylation by [18F]-fluoroacetone of the corresponding (S)-desisopropyl compound according to a known procedure. The introduction of fluorine in the isopropyl group creates a new stereogenic centre resulting in the formation of (S,S)- and (S,R)-1'-[18F]fluorocarazolol, which were separated by HPLC. Tissue distribution studies were performed in male Wistar rats. Both the (S,S)- and (S,R)-diastereomers (S.A. 500-2000 Ci/mmol; 18.5-74 TBq/mmol) showed high uptake in lung and heart, which could be blocked by pretreatment of the animals with (+/-)-propranolol. No significant differences were observed between the biodistribution of the two diastereomers. Metabolite analysis showed a rapid appearance of polar metabolites in plasma, while at 60 min postinjection 92% and 82% of the total radioactivity in lung and heart was unmetabolized 1'-[18F]fluorocarazolol. In a PET-study with male Wistar rats, the lungs were clearly visualized and the pulmonary uptake was decreased after pretreatment of the animals with (+/-)-propranolol. The heart could not be visualized. Similar results were obtained in PET-studies with lambs.

Role of G-proteins in altered beta-adrenergic responsiveness in the failing and hypertrophied myocardium

In the heart and other tissues, beta-adrenergic desensitization occurs during treatment with catecholamines. In heart failure, a strong sympathetic activation has been observed and is the cause of beta-adrenergic desensitization in this condition. On the receptor level, there is a downregulation of beta 1-adrenergic receptors as well as an uncoupling of beta 2-adrenoceptors. The latter mechanism has been related to an increased activity and gene expression of beta-ARK2 in failing myocardium leading to phosphorylation and uncoupling of receptors. In addition, an increase of inhibitory G-protein alpha-subunits (Gi alpha) has been suggested to be causally linked to adenylyl cyclase desensitization in heart failure. In contrast, the catalytic subunit of adenylyl cyclase, stimulatory G-protein alpha-subunits and beta gamma-subunits have been observed to be unchanged. Recently, evidence has been raised that increases of Gi alpha also depress adenylyl cyclase in compensated cardiac hypertrophy in monogenic and polygenic as well as in secondary hypertension. These increases of Gi alpha can suppress adenylyl cyclase in the absence of beta-adrenergic receptor downregulation. Since cardiac hypertrophy in pressure overload is a strong predictor of cardiac failure these observations indicate that adenylyl cyclase desensitization by Gi alpha could be a pathophysiologically relevant mechanism to contribute to the progression from compensated cardiac hypertrophy to heart failure.

Systolic blood pressure is related to catecholamine sensitivity in subcutaneous abdominal fat cells

A relationship between abdominal obesity and hypertension is well established. In search for an early-onset defect in adipocyte function linking these two conditions, we compared catecholamine sensitivity in subcutaneous abdominal fat cells with 24-hour systolic, mean arterial and diastolic blood pressure in 16 healthy, normotensive subjects. Clear inter-individual variations in the adipocyte lipolytic adrenoceptor sensitivity (pD2) for noradrenaline were observed in dose-response experiments (i.e., about 4 log units). An inverse and independent correlation was found between the 24-hour systolic blood pressure and pD2 for noradrenaline (r = -0.67, p < 0.01). The mean arterial blood pressure was also negatively correlated to peripheral noradrenaline sensitivity (r = -0.58, p < 0.05). However, no significant relationship between the 24-hour diastolic blood pressure and pD2 for noradrenaline was demonstrated. In conclusion we suggest that catecholamine resistance in subcutaneous fat cells may be associated with autonomic dysfunction and impaired blood pressure regulation. This finding is supported by the fact that both noradrenaline sensitivity and 24-hour systolic blood pressure also are correlated to the individual orthostatic heart rate responses, reflecting the sympathetic nervous system tone (r = 0.61, p = 0.01 and r = -0.53, p = 0.03, respectively). The relationship between noradrenaline sensitivity and systolic blood pressure may be of importance in the early development of hypertension in man.

beta-Adrenergic relaxation in mesenteric resistance arteries of spontaneously hypertensive and Wistar-Kyoto rats: the role of precontraction and intracellular Ca2+

An attenuated beta-adrenergic vasodilation of small arteries may help explain the increased peripheral resistance in hypertension. To investigate this, we compared the isoprenaline-induced relaxation of mesenteric resistance arteries of spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) using a small vessel myograph. The arteries had similar diameters, but the contractile force induced by cumulative addition of K+ (10-130 mM) was 1.3-fold higher for the SHR. The beta-adrenoceptor-mediated relaxation of arteries, precontracted with 40 mM K+, was significantly less in SHR (41 +/- 3%, n = 11) than in WKY (56 +/- 3%, n = 15, p = 0.003), and the pD2 value for isoprenaline was significantly lower in SHR (7.13 +/- 0.09 vs. 7.41 +/- 0.07, p = 0.02). In contrast, when precontracted with phenylephrine (PE, alpha 1-adrenoceptor agonist, 3-10 microM), isoprenaline relaxation was almost complete in both SHR and WKY, and the pD2 value for isoprenaline did not differ between strains. Forskolin induced complete relaxation of both precontractions. Because the beta-adrenergic relaxation of the mesenteric resistance arteries was attenuated only after K(+)-precontraction, we conclude that alterations in this precontracting mechanism in SHR rather than a defect in the beta-adrenoceptor system may provide an explanation for the decreased relaxation in these vessels. Intracellular Ca2+ measurements and a review of the literature support this conclusion.

Dose-dependent dissociation of ACE-inhibitor effects on blood pressure, cardiac hypertrophy, and beta-adrenergic signal transduction

BACKGROUND

Dose-dependent effects of ACE inhibitors on blood pressure, cardiac hypertrophy, and beta-adrenergic signal transduction were examined in an animal model with beta-adrenergic desensitization, which has been identified in failing hearts and in hypertensive cardiac hypertrophy. It is unknown whether beneficial ACE-inhibitor effects are due to an unloading of the failing heart or a reduction of neuroendocrine activation with beta-adrenergic resensitization.

METHODS AND RESULTS

Low-dose (LD, 1 mg/kg) and high-dose (HD, 25 mg/kg) fosinopril treatment was performed in spontaneously hypertensive rats (SHR) and control (WKY) rats. Myocardial norepinephrine concentrations, adenylyl cyclase activity, beta-adrenergic receptors (radioligand binding), Gs alpha (functional reconstitution), and Gi alpha (pertussis toxin labeling) were determined. Ventricular weights and blood pressures were measured. HD but not LD reduced blood pressure and left ventricular weights in SHR. Isoprenaline- and guanylylim-idodiphosphate-stimulated adenylyl cyclase activities as well as beta 1-adrenergic receptors were reduced in SHR. The catalyst and Gs alpha were unchanged, but Gi alpha and norepinephrine concentrations were increased. Both LD and HD treatments restored beta-adrenergic alteration.

CONCLUSIONS

LD treatment with ACE inhibitors restored beta-adrenergic signal transduction defects independently of regression of cardiac hypertrophy. This could contribute to the effects of ACE inhibitors in patients, who are often treated with nonhypotensive doses.

Relationships between kallikrein secretion, kallikrein excretion and beta-adrenoceptors in kidney cortical slices from neurogenic hypertensive dogs

1. Sinoaortic denervation (SAD) in dogs is characterized by an increase in blood pressure and heart rate as well as the development of renal morphological lesions similar to those observed in essential hypertension in human subjects. To assess the effect of SAD on the secretion of kallikrein kinin systems (KKS), we studied the in vitro secretion of kallikrein by renal cortical slices of normal and neurogenic hypertensive dogs (1 and 18 months after SAD). The method using renal cortical slices allowed the study of secretion of kallikrein independently of renal perfusion pressure. The number of renal beta-adrenoceptors was measured by [125I]-cyanopindolol binding. 2. SAD was associated with a marked increase in urinary kallikrein excretion at one month and a significant decrease at 18 months when compared with controls. Both changes were statistically significant (P < 0.05). Concurrently, a progressive increase in in vitro kallikrein secretion was observed (+80 +/- 10% and +179 +/- 48%, 1 and 18 months after SAD, respectively). Moreover, the cortical slices obtained from sinoaortic denervated dogs contained more kallikrein than the control cortical slices (+32 +/- 16% and +55 +/- 7%, 1 and 18 months after SAD, respectively). 3. Renal beta-adrenoceptor number significantly (P < 0.05) decreased 18 months after SAD from 18 +/- 2 to 8 +/- 3 fmol mg-1 protein without any change in affinity constant. 4. Although there was no test of association, because the number of renal beta-adrenoceptors decreased whereas kallikrein secretion increased, the present data could suggest a beta-adrenoceptor-mediated inhibition of kallikrein secretion. These results show that although the urinary kallikrein is decreased, the tissue secretory capacities are enhanced. This could suggest a renal compensatory mechanism possibly involved in tissue protection in dogs after SAD, although such a mechanism is not sufficient to reverse hypertension.

Attenuation of isoproterenol-mediated vasodilatation in blacks

BACKGROUND

The mechanism of enhanced vascular reactivity in young blacks, which may play a part in the development of hypertension, has not been defined. To determine the contribution of blunted vasodilatation mediated by beta 2-adrenergic receptors to this phenomenon, we compared forearm blood-flow responses to isoproterenol in young black and white normotensive men.

METHODS

We used venous-occlusion plethysmography to measure the responses of blood flow in the forearm to the intraarterial administration of isoproterenol (10 to 400 ng per minute) in 9 normotensive black men (mean [+/- SD] age, 31.3 +/- 8.0 years) and 13 normotensive white men (mean age, 32.9 +/- 5.6 years). Sympathetic activity in the forearm was measured simultaneously by isotope-dilution techniques.

RESULTS

Base-line blood flow in the forearm was similar in blacks and whites, but the degree of vasodilatation in response to isoproterenol was markedly lower in blacks. Isoproterenol at an infusion rate of 400 ng per minute produced a 9-fold increase in blood flow in whites but only a 3.5-fold increase in blacks (P < 0.001). The base-line rate of norepinephrine spillover in the forearm was higher in blacks (2.0 +/- 1.3 ng per minute [11.8 +/- 7.7 nmol per minute]) than in whites (0.6 +/- 0.5 ng per minute [3.5 +/- 3.0 nmol per minute], P = 0.002), but there was no difference between the groups after isoproterenol stimulation.

CONCLUSIONS

Forearm blood-flow responses to isoproterenol were markedly attenuated in normotensive blacks, indicating a blunting of vasodilatation mediated by beta 2-adrenergic receptors. Sympathetic activity in the forearm was greater in blacks than in whites, but isoproterenol-stimulated presynaptic beta 2-adrenergic responses (which facilitated norepinephrine release) did not differ significantly between blacks and whites. Our findings suggest that the mechanisms responsible for blunted vasodilatation in response to the administration of isoproterenol may contribute to enhanced vascular reactivity in blacks and may play a part in the pathogenesis of hypertension in blacks.

Treatment in hypertensive cardiac hypertrophy, I. Neuropeptide Y and beta-adrenoceptors

In the present study, we investigated serum and myocardial neuropeptide Y concentrations as measures of sympathetic activity as well as myocardial beta-adrenoceptors and beta-adrenoceptor-stimulated adenylyl cyclase activity in spontaneously hypertensive rats (SHR). SHR and control rats at 10 weeks of age were kept on oral treatment with captopril, nitrendipine, or both for 20 weeks. Treatment only slightly reduced but did not normalize blood pressure and cardiac hypertrophy in SHR. The elevated serum concentration of neuropeptide Y, the reduced number of beta-adrenoceptors, and the depressed beta-adrenoceptor-stimulated adenylyl cyclase activity were partly normalized compared with the values observed in control rats. We conclude that antihypertensive treatment, at doses that failed to normalize systolic pressure and to reverse cardiac hypertrophy completely, is able to reduce sympathetic activity in SHR, thereby resensitizing the depressed beta-adrenoceptor-adenylyl cyclase system.

Selective downregulation of rat cardiac beta 1-adrenoceptors by cyclosporine A: prevention by diltiazem or angiotensin-converting enzyme inhibitors

OBJECTIVES

This study attempted to determine whether long-term treatment with cyclosporine A in rats affects cardiac beta 1-adrenoceptors and whether this can be prevented by angiotensin-converting enzyme inhibitors or calcium-entry blocking agents.

BACKGROUND

In the transplanted human heart the density of beta 1-adrenoceptors decreases with time after transplantation, whereas that of beta 2-adrenoceptors does not. Because heart transplant recipients are treated with cyclosporine A, we studied whether administration of cyclosporine A in rats might cause this beta 1-adrenoceptor downregulation.

METHODS

We performed two studies. First, we treated groups of 10 male normotensive Wistar rats orally with 30 mg/kg body weight per day of cyclosporine A, 10 mg/kg per day of enalapril and 60 mg/kg per day of diltiazem, alone or in combination, for 6 weeks each. Second, we treated groups of 15 male normotensive Wistar rats orally with 15 mg/kg per day of cyclosporine A and 10 mg/kg per day of lisinopril, alone or in combination, for 6 weeks each. At the end of each treatment regimen, cardiac beta-adrenoceptor density and subtype distribution were assessed by (-)-[125I]iodocyanopindolol binding.

RESULTS

Both doses of cyclosporine A caused a significant decrease in cardiac beta 1-adrenoceptor density without affecting beta 2-adrenoceptor density. Although diltiazem and the angiotensin-converting enzyme inhibitors alone did not affect cardiac beta-adrenoceptors, they prevented the cyclosporine A-induced downregulation of beta 1-adrenoceptors.

CONCLUSIONS

In normotensive Wistar rats, cyclosporine A causes a significant decrease in cardiac beta 1-adrenoceptors without affecting beta 2-adrenoceptors. This can be prevented by diltiazem or angiotensin-converting enzyme inhibitors. In heart transplant recipients, who undergo long-term treatment with cyclosporine A, there is a very similar beta 1-adrenoceptor down-regulation with time after transplantation. Thus, administration of cyclosporine A may cause these beta-adrenoceptor subtype alterations.